54 GEOLOGY OF THE EUREKA UlSTEICT. 



Orthis trieenaria, 0. testudhiaria, and 0. perveta, characteristic forms in iNew 

 York and Wisconsin. A marked feature of this upper horizon is the [)res- 

 ence of tlie genus BeceptacuUtcs, tlii-ee species having- been identified. Im- 

 mense numbers of specimens of one of them, B. maiiDniUaris, are found 

 throughout the beds with a vertical range of several hundred feet, and are 

 alnindant where all other fossils are wanting. Graptolitcs, in the Pogonip 

 epoch at Eureka, are represented by a single undetermined sjjecies, which, 

 according to Mr. C. D. Walcott, resembles closely G. Ufidm. 



Eureka Quartzite.— The name of the district has been employed to desig- 

 nate this formation, as during the progress of the survey the quartzite was 

 determined for the first time as a distinct geological epoch and its strati- 

 graphical i^osition clearly defined. Up to this time the occurrence of a 

 broad belt of quartzite lying between two massive bodies of Silurian lime- 

 stone had never been recognized. Moreover, nowhere else in the Great 

 Basin has the formation been so carefully studied. It lies superimposed 

 directly on the Pogonip limestone, and where the upper beds of the latter 

 epoch are exposed they are frequently capped by a greater or less thickness 

 of the quartzite, as is well shown on Caribou Hill and McCoy's Ridge. 

 Again, the position of the Eureka quartzite is clearly brought out by the 

 patches of quartzite left by erosion upon the massive Pogonip beds of Fish 

 Creek Mountains. No horizon is more marked in its physical features 

 than the Eureka quartzite. Besides its frequent occurrence as a capping 

 rock, its snow-white color, and its tendency to fracture in mural-like escarp- 

 ments render it easily recognizable wherever it occurs. 



The Eureka quartzite is made np almost entirely of siliceous grains firmly 

 compacted together. It possesses a granular texture and a vitreous luster, 

 and for the most part is free from partings parallel to the planes of bedding. 

 At the base of the formation the quartzite is colored red and gray by iron, 

 but it rapidly passes into white, with an occasional bluish or ^Jurplish tinge, 

 frequently presenting a mottled coloring. In general it is exceptionally free 

 from seams or patches of ferruginous material, its purity and uniformity of 

 composition and marble-like appearance being a marked featin-e of the hor- 

 izon. In one or two places it shows a brecciated appeaiance, with fine, 

 cherty masses, notably on Hoosac Mountain. In the neighborhood of 



